Method of forming thick silica-based film

ABSTRACT

A process for easily and efficiently forming a silica-based coating film having a film thickness of from 0.5 to 5 μm on a substrate, such a coating film, a coating fluid to be used for forming such a coating film, and a process for producing such a coating fluid, are presented.  
     The present invention provides a process for forming a silica-based coating film, characterized by heating a reaction mixture comprising a silicon compound (A) represented by Si (OR) 4  and/or a silicon compound (B) represented by R 1   n Si (OR 2 ) 4−n  (wherein n is an integer of from 1 to 3), an alcohol (C) represented by R 3 CH 2 OH and oxalic acid (D) in specific ratios, at a temperature of from 50 to 180° C. in the absence of water, to form a solution of a polysiloxane having a number average molecular weight, as calculated as polystyrene, of from 2,000 to 15,000, applying a coating fluid containing such a solution on a substrate surface, and thermally curing a coating film obtained by such coating, at a temperature of from 80 to 600° C., and such a coating film having a film thickness of from 0.5 to 5 μm, a coating fluid to be used for such a coating film, and a process for producing such a coating fluid.

TECHNICAL FIELD

[0001] The present invention relates to a process for forming asilica-based coating film, whereby a thick film can be formed. Moreparticularly, the present invention relates to a process for forming athick film suitable for application to electronic materials such as aplanarization film, an interlayer insulation film, a protective film, apassivation film, etc., by using a silica-based coating material capableof forming a thick film. And, it relates to such a silica-based coatingmaterial and a process for its production.

BACKGROUND ART

[0002] Heretofore, a silica-based coating material has been used invarious fields. For example, in a semiconductor device, it has been usedas an insulation film between a semiconductor substrate and a metalwiring layer such as an aluminum wiring layer, or between metal wirings.Further, it has been used as a PN junction semiconductor formed on asemiconductor substrate, or as a protective film for various elementssuch as a capacitor element, a resistor element, etc.

[0003] Further, when a metal wiring layer or the like is formed on asemiconductor substrate, irregularities are formed by the metal wiringlayer or the like on the semiconductor substrate. If it is attempted toform a metal wiring layer or the like on such an irregular surface,breaking of wire is likely to result due to the difference in level ofthe irregularities. By forming a silica-based insulation film asmentioned above, on such an irregular surface formed by a metal wiringlayer and various elements, a role of planarization is also performed.

[0004] With respect to the silica-based coating film used in theabove-mentioned fields, it is common to form the silica-based coatingfilm by a vapor phase growing method such as a CVD method or asputtering method, or a coating method for forming the silica-basedcoating film by means of a coating composition for forming asilica-based coating film.

[0005] As a coating fluid to form such a silica-based coating film, asiloxane polymer has been used. A polyalkylsilsesquioxane precursor asone of precursors of such a siloxane polymer, is disclosed, for example,in JP-A-63-241076 or JP-A-3-126612. However, the conventional method isa method wherein an alkylalkoxysilane is hydrolyzed to form analkylsiloxane polymer. In such a method, it is not easy to control thehydrolysis or the reaction for polycondensation. Further, if such asolution is highly concentrated to such an extent that a thick film canbe formed by a single coating operation, the storage stability tends tobe poor. Accordingly, in order to form a thick film, it has been commonto carry out coating a few times for overcoating.

[0006] Further, a method wherein a polysilazane is thermally treated andused as a silicon oxynitride film (JP-A-62-88327), or a method wherein apolysilazane is thermally treated in a steam atmosphere and used asconverted to a silicon oxide film. In such methods wherein apolysilazane is employed, it is possible to form a thick film by asingle coating operation, but there is a problem such that ammonia or anamine will be generated during the thermal treatment and willcontaminate the wiring material, etc.

DISCLOSURE OF THE INVENTION

[0007] The present invention is intended to provide a process for simplyand efficiently forming a silica-based coating film having a filmthickness of from 0.5 to 5 μm on various substrates, such a coatingfilm, a stable coating fluid as an industrial product to be used forsuch a silica-based coating film, and a process for efficientlyproducing such a coating fluid.

[0008] In the present invention, a silica-based coating film of from 0.5to 5 μm can be formed on a substrate surface by forming a reactionmixture comprising a silicon compound (A) represented by the formula(1):

Si(OR)₄  (1)

[0009] (wherein R is an alkyl group having from 1 to 5 carbon atoms)and/or a silicon compound (B) represented by the formula (2):

R¹ _(n)Si(OR²)_(4−n)  (2)

[0010] (wherein R¹ is an unsubstituted or substituted alkyl group havingfrom 1 to 5 carbon atoms, an alkenyl group or an aryl group, R² is analkyl group having from 1 to 5 carbon atoms, and n is an integer of from1 to 3), an alcohol (C) represented by the formula (3):

R³CH₂OH  (3)

[0011] (wherein R³ is a hydrogen atom, or an unsubstituted orsubstituted alkyl group having from 1 to 12 carbon atoms), and oxalicacid (D), in such ratios that the alcohol (C) is from 0.5 to 100 molsper mol of all alkoxy groups contained in the silicon compounds (A) and(B), and the oxalic acid (D) is from 0.2 to 2 mols per mol of all alkoxygroups in the silicon compounds (A) and (B), and while maintaining themixture at a SiO₂ concentration of from 0.5 to 11% as calculated fromsilicon atoms in the mixture, by means of the alcohol (C), heating thereaction mixture at a temperature of from 50 to 180° C. until the totalremaining amount of the silicon compounds (A) and (B) in the reactionmixture becomes to be not more than 5 mol %, to form a solution of apolysiloxane having a number average molecular weight, as calculated aspolystyrene, of from 2,000 to 15,000 thereby formed, then applying acoating fluid containing the solution of the polysiloxane to a substratesurface, and thermally curing a coating film obtained by theapplication, at a temperature of from 80 to 600° C.

[0012] In the present invention, as a silicon compound, the siliconcompound (A) and/or the silicon compound (B) is used. Here, the siliconcompound (A) and/or the silicon compound (B) includes the followingthree modes. 1) The silicon compound (A), 2) the silicon compound (B),and 3) the silicon compound (A) and the silicon compound (B).

[0013] The solution of a polysiloxane of the present invention istransparent and contains no gelled polysiloxane. A large amount of thealcohol (C) and a relatively large amount of oxalic acid (D) arecoexistent, but the silicon compound (A) and/or the silicon compound (B)is heated in the reaction mixture having no water added, and thispolysiloxane is not one formed by condensation of a hydrolyzate of thesilicon compound (A) and/or the silicon compound (B). In a case where apolysiloxane is formed from an alkoxy silane by a method of hydrolysisin an alcohol solvent, it is likely that the liquid tends to haveturbidity or a non-uniform polysiloxane tends to form as the hydrolysisproceeds. However, with the reaction mixture by the present invention,no such phenomenon will take place.

[0014] Of the polysiloxane obtained by the present invention, thechemical structure is complex and can hardly be specified. However, itis considered that a polysiloxane having such a polymerization degree asto form a solution, is formed even if it has a branched structure, asthe polymerization takes place by reacting the alcohol (C) to anintermediate formed by the reaction of the silicon compound (A) and/orthe silicon compound (B) with oxalic acid (D).

[0015] The polysiloxane obtained by the present invention has a numberaverage molecular weight, as calculated as polystyrene, of from 2,000 to15,000, preferably from 2,000 to 8,000. If the molecular weight issmaller than 2,000, shrinkage of the film during the film formationtends to be large, and if the film thickness is increased beyond 0.5 μm,cracking will be likely. Further, if the molecular weight exceeds15,000, the obtained polysiloxane tends to be of a high molecularweight, whereby a problem such as gelation or high viscosity of thepolysiloxane solution is likely to result, and the storage stabilitytends to be poor.

[0016] By heating the coating film containing the above-mentionedpolysiloxane solution coated on the substrate, the curing reaction ofthe polysiloxane in the coating film and removal of the volatilecomponent from the coating film will proceed thereby to form aninsoluble coating film excellent in transparency, which intimatelyadheres to the substrate surface and which is thick and excellent in thetransparency and has a high cracking limit.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] Examples of the alkyl groups R and R² contained in the formulae(1) and (2) include methyl, ethyl, propyl, butyl and pentyl. Preferredexamples of the silicon compound (A) include tetramethoxysilane,tetraethoxysilane, tetrapropoxysilane and tetrabutoxysilane. Among them,tetramethoxysilane or tetraethoxysilane is, for example, particularlypreferred.

[0018] R¹ in the formula (2) may, for example, be methyl, ethyl, propyl,butyl, pentyl, phenyl or vinyl. Preferred examples of the siliconcompound (B) include, in a case where n in the formula (2) is an integerof 1, methyltrimethoxysilane, ethyltrimethoxysilane,propyltriethoxysilane, butyltriethoxysilane, phenyltrimethoxysilane,vinyltrimethoxysilane, methyltriethoxysilane, ethyltriethoxysilane,propyltriethoxysilane, butyltriethoxysilane, phenyltriethoxysilane andvinyltriethoxysilane; in a case where n in the formula (2) is an integerof 2, dimethyldimethoxysilane, diethyldimethoxysilane,dipropyldiethoxysilane, dibutyldiethoxysilane, diphenyldimethoxysilane,divinyldimethoxysilane, dimethyldiethoxysilane, diethyldiethoxysilane,dipropyldiethoxysilane, dibutyldiethoxysilane, diphenyldiethoxysilane,and divinyldiethoxysilane; and in a case where n in the formula (2) isan integer of 3, trimethylmethoxysilane, triethylmethoxysilane,tripropylmethoxysilane, tributylmethoxysilane, triphenylmethoxysilane,trivinylmethoxysilane, trimethylethoxysilane, triethylethoxysilane,tripropylethoxysilane, tributylethoxysilane, triphenylethoxysilane, andtrivinylethoxysilane.

[0019] Among these, in a case where the silicon compound (B) is usedalone without combining it with the silicon compound (A), more preferredis methyltrimethoxysilane, ethyltrimethoxysilane, propyltriethoxysilane,butyltriethoxysilane, phenyltrimethoxysilane, vinyltrimethoxysilane,methyltriethoxysilane, ethyltriethoxysilane, propyltriethoxysilane,butyltriethoxysilane, phenyltriethoxysilane or vinyltriethoxysilane, asexemplified in the case where n is an integer of 1 in the formula (2).Among them, particularly preferred is methyltriethoxysilane.

[0020] Further, in a case where the silicon compound (B) is used incombination with the silicon compound (A), methyltriethoxysilane,dimethyldiethoxysilane, ethyltriethoxysilane, diethyldiethoxysilane,γ-metacryloxypropyltrimethoxysilane, vinyltriethoxysilane,divinyldiethoxysilane and phenyltriethoxysilane are, for example,particularly preferred. Such silicon compounds (B) may be used alone orin combination of two or more of them.

[0021] Examples of the unsubstituted alkyl group R³ contained in theformula (3) may be methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyland octyl, and examples of the substituted alkyl group R³ includehydroxymethyl, methoxymethyl, ethoxymethyl, hydroxyethyl, methoxyethyland ethoxyethyl. Preferred examples of the alcohol (C) include methanol,ethanol, propanol, n-butanol, ethylene glycol monomethyl ether, andethylene glycol monoethyl ether. These may be used alone or incombination of two or more of them. Among them, particularly preferredis ethanol.

[0022] If oxalic acid (D) is used in an amount smaller than 0.2 mol permol of all alkoxy groups contained in the silicon compounds (A) and (B),the molecular weight of the resulting polysiloxane tends to be low, andthe hardness of the resulting film tends to be low. On the contrary, ifthe oxalic acid (D) is used in an amount larger than 2 mols per mol ofall alkoxy groups contained in the silicon compounds (A) and (B), theresulting polysiloxane-containing solution contains a relatively largeamount of oxalic acid (D), and from such a solution, a film having theintended performance can not be obtained. It is particularly preferredto use oxalic acid (D) in an amount of from 0.25 to 1 mol per mol of allalkoxy groups contained in the silicon compounds (A) and (B).

[0023] In the preparation of the polysiloxane solution, in addition tothe silicon compound (A), the silicon compound (B), the alcohol (C) andthe oxalic acid (D), an alkoxysilane which is not used in the abovesilicon compounds (A) and (B), may be incorporated as a modifier, as thecase requires. Preferred examples of such a modifier includeγ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane,γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropyltriethoxysilane,γ-methacryloxypropyltrimethoxysilane andγ-methacryloxypropyltriethoxysilane. These modifiers can lower thetemperature for curing the coating film on the substrate and improve theadhesion of the film to the substrate.

[0024] The reaction mixture comprising the silicon compound (A) and/orthe silicon compound (B), the alcohol (C) and the oxalic acid (D), canbe formed by mixing them, or by further adding the above-mentionedmodifier to them. To this reaction mixture, water may not be added. And,this reaction mixture is preferably heated as a reaction mixture in aliquid state. For example, it is preferred to heat it as a reactionmixture in a solution state obtained by preliminarily adding anddissolving the oxalic acid (D) in the alcohol (C) to obtain a solutionand then mixing the silicon compound (A) and/or silicon compound (B),the above modifier, etc., or by heating it as a reaction mixture in asolution state obtained by mixing a solution having oxalic acid (D)preliminarily added and dissolved in the alcohol (C), into a mixture ofthe silicon compound (A) and/or silicon compound (B) and theabove-mentioned modifier. Usually, the reaction mixture comprising thesilicon compound (A), the silicon compound (B), the alcohol (C) and theoxalic acid (D) in the above-mentioned ratios, contains silicon atomscontained therein at a concentration of from 0.5 to 11 wt % ascalculated as SiO₂. Also in a case where the above modifier iscontained, such a modifier is contained so that the silicon atomscontained therein will have a concentration of from 0.5 to 11% ascalculated as SiO₂. And, during the heating of such a reaction mixture,such a reaction mixture is maintained at the above-mentioned SiO₂concentration, and absence of water is also maintained. This heating canbe carried out at a liquid temperature of from 50 to 180° C. in a usualreactor. Preferably, it is carried out, for example, in a closedcontainer or under reflux so that evaporation or dissipation of theliquid from the reactor will not take place.

[0025] If the heating to form the polysiloxane is carried out at atemperature lower than 50° C., a liquid having a turbidity or containinginsoluble substances, is likely to be formed. Accordingly, this heatingis carried out at a temperature higher than 50° C., and it can becompleted in a short time as the temperature is high. However, heatingat a temperature higher than 180° C. is not efficient as it brings aboutno additional merit. The heating time is not particularly limited, andfor example, it is usually at a level of 8 hours at 50° C. or at a levelof 3 hours under reflux at 78° C. Usually, with apolysiloxane-containing solution in which the silicon compound (A) and(B) remain more than 5 mol %, based on the total amount of these siliconcompounds charged, if such a solution is coated on the substrate surfaceand then the coated film is thermally cured at a temperature of from 80to 600° C., the obtained film is likely to have pinholes, or a filmhaving a sufficient hardness can hardly be obtained.

[0026] The polysiloxane solution obtained by the above heating can beused by itself as a coating fluid for the next coating step. However, asthe case requires, a liquid obtained by concentration or dilution may beused as a coating fluid, a liquid obtained by substituting other solventmay be used as a coating fluid, or a liquid obtained by incorporating adesired additive may be used as a coating fluid. As an example of suchan additive, solid inorganic fine particles such as colloidal fineparticles, a metal salt or a metal compound may be mentioned, and suchan additive is preferred to control the hardness of the film, theadhesion to the substrate, the refractive index, etc. The coating fluidto be used for the coating step is preferably a liquid which containssilicon atoms derived from the transparent solution of theabove-mentioned polysiloxane in an amount of from 0.5 to 18 wt % ascalculated as SiO₂. If this SiO₂ concentration is smaller than 0.5 wt %,the thickness of film to be formed by one coating operation tends to bethin, and if this concentration exceeds 18 wt %, the storage stabilityof this solution tends to be inadequate. The SiO₂ concentration of thiscoating fluid is particularly preferably from 2 to 15%.

[0027] The above solution of the polysiloxane can be coated on asubstrate by a usual method such as a dipping method, a spin coatingmethod, a brush coating method, a roll coating method or a flexoprintingmethod. The coating on the substrate by the above solution of thepolysiloxane is characterized in that a sufficiently thick film can beformed by a single coating operation, but if necessary, it may be coatedin a plurality of times.

[0028] The coating film formed on the substrate may be thermally curedas it is. However, it may preliminarily be dried at a temperature offrom room temperature to 80° C., preferably from 50 to 80° C. and thenheated at a temperature of from 80 to 600° C., preferably from 80 to400° C. The time for this heating may sufficiently be from about 5 to 60minutes. If this heating is lower than 80° C., the hardness, chemicalresistance, etc. of the resulting coating film tend to be inadequate. Ata high temperature exceeding 600° C., elimination of organic groups islikely to take place, whereby the coating film is likely to have cracks,or the coating film is likely to be peeled, such being undesirable. Suchheating can be carried out by a usual method, for example, by using ahot plate, an oven or a belt furnace.

[0029] The thickness of the film after curing is usually adjusted to befrom 0.01 to 3.0 μm. The silica-based coating film obtainable by thepresent invention can be used as a relatively thick film of from 0.5 to5 μm which is effective for the purpose of planarization to eliminatethe difference in level of the primer.

[0030] Now, the present invention will be described specifically withreference to Examples and Comparative Examples, but it should beunderstood that the present invention is by no means restricted by suchExamples.

[0031] Here, with respect to the molecular weight of the polysiloxanesolution in each Example, the number average molecular weight, ascalculated as polystyrene, was obtained in accordance with a GPC methodby a high performance liquid chromatography. The measuring conditionsare shown below.

[0032] Measuring conditions by the GPC method

[0033] Eluent: THF

[0034] Column: KF-804L, manufactured by Shodex Company

[0035] Flow rate: 1 mL/min

[0036] Standard substance: polystyrene (210000, 70600, 28600, 9860,2960, 580)

[0037] Detector: RI

[0038] Sample concentration: 1 wt %

EXAMPLE 1

[0039] Into a four-necked reactor flask equipped with a refluxcondenser, 61.2 g of ethanol was charged, and 18.0 g of oxalic acid wasgradually added to this ethanol with stirring, to prepare an ethanolsolution of oxalic acid. Then, into this solution, 20.8 g oftetraethoxysilane was dropwise added. After completion of the dropwiseaddition, this solution was heated to a reflux temperature, and heatingwas continued for 5 hours under reflux, followed by cooling to obtain apolysiloxane solution (L-1), having a solid content concentration of 6wt %.

[0040] This solution was analyzed by gas chromatography, whereby noalkoxysilane monomer was detected. Further, the molecular weight of thissolution was 2,800, as the number average molecular weight, ascalculated as polystyrene.

EXAMPLE 2

[0041] Into a four-necked reactor flask equipped with a refluxcondenser, 64.9 g of ethanol was charged, and 15.8 g of oxalic acid wasgradually added to this ethanol with stirring, to prepare an ethanolsolution of oxalic acid. Then, into this solution, a mixture comprising10.4 g of tetraethoxysilane and 8.9 g of methyltriethoxysilane, wasdropwise added. After completion of the dropwise addition, this solutionwas heated to its reflux temperature, and heating was continued for 5hours under reflux, followed by cooling to obtain a polysiloxanesolution (L-2), having a solid content concentration of 6 wt %.

[0042] This solution was analyzed by gas chromatography, whereby noalkoxysilane monomer was detected. Further, the molecular weight of thissolution was 5,600, as the number average molecular weight, ascalculated as polystyrene.

EXAMPLE 3

[0043] Into a four-necked reactor flask equipped with a refluxcondenser, 50.7 g of ethanol was charged, and 21.6 g of oxalic acid wasgradually added to this ethanol with stirring, to prepare an ethanolsolution of oxalic acid. Then, into this solution, a mixture comprising6.3 g of tetraethoxysilane and 21.4 g of methyltriethoxysilane, wasdropwise added. After completion of the dropwise addition, this solutionwas heated to its reflux temperature, and heating was continued for 5hours under reflux, followed by cooling to obtain a polysiloxanesolution (L-3), having a solid content concentration of 9 wt %.

[0044] This solution was analyzed by gas chromatography, whereby noalkoxysilane monomer was detected. Further, the molecular weight of thissolution was 6,000, as the number average molecular weight, ascalculated as polystyrene.

EXAMPLE 4

[0045] Into a four-necked reactor flask equipped with a refluxcondenser, 53.0 g of ethanol was charged, and 20.3 g of oxalic acid wasgradually added to this ethanol with stirring, to prepare an ethanolsolution of oxalic acid. Then, into this solution, 26.8 g ofmethyltriethoxysilane was dropwise added. After completion of thedropwise addition, this solution was heated to its reflux temperature,and heating was continued for 5 hours under reflux, followed by coolingto obtain a polysiloxane solution (L-4), having a solid contentconcentration of 9 wt %.

[0046] This solution was analyzed by gas chromatography, whereby noalkoxysilane monomer was detected. Further, the molecular weight of thissolution was 6,200, as the number average molecular weight, ascalculated as polystyrene.

EXAMPLE 5

[0047] Into a four-necked reactor flask equipped with a refluxcondenser, 64.3 g of ethanol was charged, and 15.8 g of oxalic acid wasgradually added to this ethanol with stirring, to prepare an ethanolsolution of oxalic acid. Then, into this solution, a mixture comprising10.4 g of tetraethoxysilane and 9.5 g of vinyltriethoxysilane, wasdropwise added. After completion of the dropwise addition, this solutionwas heated to its reflux temperature, and heating was continued for 5hours under reflux, followed by cooling to obtain a polysiloxanesolution (L-5), having a solid content concentration of 9 wt %.

[0048] This solution was analyzed by gas chromatography, whereby noalkoxysilane monomer was detected. Further, the molecular weight of thissolution was 4,300, as the number average molecular weight, ascalculated as polystyrene.

EXAMPLE 6

[0049] Into a four-necked reactor flask equipped with a refluxcondenser, 61.8 g of ethanol was charged, and 15.8 g of oxalic acid wasgradually added to this ethanol with stirring, to prepare an ethanolsolution of oxalic acid. Then, into this solution, a mixture comprising10.4 g of tetraethoxysilane and 12.0 g of phenyltriethoxysilane, wasdropwise added. After completion of the dropwise addition, this solutionwas heated to its reflux temperature, and heating was continued for 5hours under reflux, followed by cooling to obtain a polysiloxanesolution (L-6), having a solid content concentration of 6 wt %.

[0050] This solution was analyzed by gas chromatography, whereby noalkoxysilane monomer was detected. Further, the molecular weight of thissolution was 4,700, as the number average molecular weight, ascalculated as polystyrene.

EXAMPLE 7

[0051] Into a four-necked reactor flask equipped with a refluxcondenser, 62.3 g of ethanol was charged, and 17.3 g of oxalic acid wasgradually added to this ethanol with stirring, to prepare an ethanolsolution of oxalic acid. Then, into this solution, 19.8 g oftetraethoxysilane and 0.6 g of trimethylethoxysilane were dropwiseadded. After completion of the dropwise addition, this solution washeated to its reflux temperature, and heating was continued for 5 hoursunder reflux, followed by cooling to obtain a polysiloxane solution(L-21) having a solid content concentration of 6 wt %.

[0052] This solution was analyzed by gas chromatography, whereby noalkoxysilane monomer was detected. Further, the molecular weight of thissolution was 3,100, as the number average molecular weight, ascalculated as polystyrene.

EXAMPLE 8

[0053] Into a four-necked reactor flask equipped with a refluxcondenser, 62.7 g of ethanol was charged, and 17.1 g of oxalic acid wasgradually added to this ethanol with stirring, to prepare an ethanolsolution of oxalic acid. Then, into this solution, 18.7 g oftetraethoxysilane and 1.5 g of dimethyldiethoxysilane were dropwiseadded. After completion of the dropwise addition, this solution washeated to its reflux temperature, and heating was continued for 5 hoursunder reflux, followed by cooling to obtain a polysiloxane solution(L-22) having a solid content concentration of 6 wt %.

[0054] This solution was analyzed by gas chromatography, whereby noalkoxysilane monomer was detected. Further, the molecular weight of thissolution was 3,800, as the number average molecular weight, ascalculated as polystyrene.

COMPARATIVE EXAMPLE 1

[0055] Into a four-necked reactor flask equipped with a refluxcondenser, 71.9 g of ethanol and 20.8 g of tetraethoxysilane were addedand uniformly mixed. Then, to this solution, 7.2 g of water and, as acatalyst, 0.1 g of concentrated nitric acid (60 wt % nitric acid) wereadded, and stirring was continued for 30 minutes, to obtain apolysiloxane solution (L-7) having a solid content concentration of 6 wt%.

COMPARATIVE EXAMPLE 2

[0056] Into a four-necked reactor flask equipped with a refluxcondenser, 74.3 g of ethanol, 10.4 g of tetraethoxysilane and 8.9 g ofmethyltriethoxysilane, were added and uniformly mixed. Then, to thissolution, 6.3 g of water and, as a catalyst, 0.1 g of concentratednitric acid (60 wt % nitric acid) were added, and stirring was continuedfor 30 minutes, to obtain a polysiloxane solution (L-8) having a solidcontent concentration of 6 wt %.

COMPARATIVE EXAMPLE 3

[0057] Into a four-necked reactor flask equipped with a refluxcondenser, 66.4 g of ethanol, 6.3 g of tetraethoxysilane and 21.4 g ofmethyltriethoxysilane, were added and uniformly mixed. Then, to thissolution, 5.8 g of water and, as a catalyst, 0.1 g of concentratednitric acid (60 wt % nitric acid) were added, and stirring was continuedfor 30 minutes, to obtain a polysiloxane solution (L-9) having a solidcontent concentration of 9 wt %.

COMPARATIVE EXAMPLE 4

[0058] Into a four-necked reactor flask equipped with a refluxcondenser, 67.7 g of ethanol and 26.8 g of methyltriethoxysilane wereadded and uniformly mixed. Then, to this solution, 5.4 g of water and,as a catalyst, 0.1 g of concentrated nitric acid (60 wt % nitric acid)were added, and stirring was continued for 30 minutes, to obtain apolysiloxane solution (L-10) having a solid content concentration of 9wt %.

COMPARATIVE EXAMPLE 5

[0059] Into a four-necked reactor flask equipped with a refluxcondenser, 73.7 g of ethanol, 10.4 g of tetraethoxysilane and 9.5 g ofvinyltriethoxysilane, were added and uniformly mixed. Then, to thissolution, 6.3 g of water and, as a catalyst, 0.1 g of concentratednitric acid (60 wt % nitric acid) were added, and stirring was continuedfor 30 minutes, to obtain a polysiloxane solution (L-11) having a solidcontent concentration of 6 wt %.

COMPARATIVE EXAMPLE 6

[0060] Into a four-necked reactor flask equipped with a refluxcondenser, 71.2 g of ethanol, 10.4 g of tetraethoxysilane and 12.0 g ofphenyltriethoxysilane, were added and uniformly mixed. Then, to thissolution, 6.3 g of water and, as a catalyst, 0.1 g of concentratednitric acid (60 wt % nitric acid) were added, and stirring was continuedfor 30 minutes, to obtain a polysiloxane solution (L-12) having a solidcontent concentration of 6 wt %.

COMPARATIVE EXAMPLE 7

[0061] Into a four-necked reactor flask equipped with a refluxcondenser, 57.9 g of ethanol was charged, and 18.0 g of oxalic acid wasgradually added to this ethanol with stirring, to prepare an ethanolsolution of oxalic acid. Then, into this solution, a mixture comprising4.2 g of tetraethoxysilane and 19.9 g of hexyltriethoxysilane, wasdropwise added. After completion of the dropwise addition, this solutionwas heated to its reflux temperature, and heating was continued for 5hours under reflux, followed by cooling, to obtain a polysiloxanesolution (L-13) having a solid content concentration of 6 wt %.

[0062] This solution was analyzed by gas chromatography, whereby noalkoxysilane monomer was detected. Further, the molecular weight of thissolution was 1,800, as the number average molecular weight, ascalculated as polystyrene.

EVALUATION EXAMPLE 1

[0063] The polysiloxane solutions (L-1) to (L-13) and (L-21) to (L-22)were left to stand over three months at 23° C. as closed in glasscontainers. During the period, the presence or absence of turbidity orprecipitation formed in the containers was observed, whereby the resultsas shown in Table 1 were obtained. In the same Table, symbol ◯ indicatesthat no change was observed upon expiration of three months as beingleft to stand as mentioned above, symbol Δ indicates that turbidityformed upon expiration of one month as being left to stand as mentionedabove, and symbol X indicates that turbidity formed within two weeks asbeing left to stand as mentioned above. In the same Table, (L-1) to(L-7), (L-13) and (L-21) to (L-22) showed good stability, while (L-9)underwent gelation after 10 days, and (L-10) showed turbidity at thetime of preparation of the solution. Further, (L-11) showed turbidityafter 10 days, and (L-12) showed white precipitates after 5 days.

[0064] Thus, the results in Table 1 showed that the polysiloxanesolutions of Examples had good stability as compared with thepolysiloxane solutions prepared by a hydrolytic method.

EVALUATION EXAMPLE 2

[0065] On silicon substrates, the polysiloxane solutions of (L-1) to(L-8), (L-13) and (L-21) to (L-22) were coated by a spin coater and thendried at 80° C. to form coating films on the silicon substrates,whereupon the coating film surfaces were observed to test thefilm-forming properties of these solutions. The results are shown inTable 1. In the same Table, symbol ◯ indicates that the film was auniform coating film, symbol Δ indicates that pinholes were partiallyobserved in the coating film, and symbol X indicates that cissing wasobserved in the coating film.

[0066] The results of Table 1 indicate that the polysiloxane solutions(L-1) to (L-8), (L-13) and (L-21) to (L-22) showed good film-formingproperties, while with the polysiloxane solutions (L-9) to (L-12), thefilm-forming properties were insufficient. TABLE 1 Film forming SolutionStability property L-1 ◯ ◯ L-2 ◯ ◯ L-3 ◯ ◯ L-4 ◯ ◯ L-5 ◯ ◯ L-6 ◯ ◯ L-7*⁾◯ ◯ L-8*⁾ Δ ◯ L-9*⁾ X Δ L-10*⁾ X X L-11*⁾ X Δ L-12*⁾ X X L-13*⁾ ◯ ◯ L-21◯ ◯ L-22 ◯ ◯

EVALUATION EXAMPLE 3

[0067] The polysiloxane solutions (L-1) to (L-8), (L-13) and (L-21) to(L-22) which showed good film-forming properties in Evaluation Example1, were, respectively, spin-coated on substrates to form coating films,and then, these coating films were dried at 80° C. for 5 minutes on ahot plate and then heated at 300° C. in a calcination furnace, to formcoating films on the substrate surfaces. Then, with respect to theobtained coating films, the pencil hardness, the maximum filmthicknesses of the coating films obtained by a single coating operation,and the transmittance, were measured by the following methods.

[0068] Pencil hardness: in accordance with the method prescribed in JISK5400.

[0069] Maximum film thickness: the above-mentioned polysiloxane solutionwas concentrated by a rotary evaporator so that the solid contentconcentration would be from 10 to 15 wt %, to obtain a coating fluid.The maximum film thickness where no cracking resulted on a substrate,was measured. The film thickness was measured in such a manner that acutting mark was imparted by a cutter to the coating film after drying,followed by thermal curing, and with respect to the obtained coatingfilm, the difference in level was measured by means of a Tallis stepmanufactured by Rank Taylor Hobson Company.

[0070] Transmittance: A coating film having a film thickness of 0.3 μm,was formed on a quartz substrate by using the above-mentionedpolysiloxane solution, and the transmittance within a wavelength rangeof from 800 to 200 nm was measured by means of a spectrophotometerUV3100PC, manufactured by Shimadzu Corporation.

[0071] The results of these measurements are shown in Table 2.

[0072] The maximum film thickness of the coating fluid (L-7) obtained bythe hydrolytic method of Comparative Example 1 was 0.3 μm, while themaximum film thickness of the coating fluid (L-1) obtained in Example 1was 0.8 μm. The maximum film thicknesses of the coating fluids (L-21)and (L-22) obtained in Examples 7 and 8, were 1.2 and 1.6 μm,respectively. Further, with the coating fluids (L-2) to (L-6) obtainedin Examples 2 to 6, coating films having a film thickness of two or moreμm, free from cracks and having high transparency, high hardness andgood flatness, were obtained. Further, with the coating fluid (L-13)having a number average molecular weight of 1,800, obtained inComparative Example 7, the maximum film thickness was 0.4 μm, althoughthe transparency and flatness were good. The transmittances of thepolysiloxane solutions obtained in Examples 1 to 6 and Examples 7 and 8,were all at least 90%, and the obtained coating films were found to haveexcellent transparency. TABLE 2 Pencil Maximum film Solution hardnessthickness (μm) Transmittance L-1 8H 0.8 At least 90% L-2 7H 2.4 At least90% L-3 7H >3.0 At least 90% L-4 7H >3.0 At least 90% L-5 8H 2.5 Atleast 90% L-6 7H 2.5 At least 90% L-7*⁾ 7H 0.3 At least 90% L-8*⁾ 6H 0.8At least 90% L-13*⁾ 4H 0.4 At least 90% L-21 8H 1.2 At least 90% L-22 7H1.6 At least 90%

INDUSTRIAL APPLICABILITY

[0073] According to the present invention, a coating fluid canefficiently be prepared simply by mixing the starting material andcarrying out heat treatment only once, and this coating fluid has astability such that it is durable in storage at least three months, witha preferred one, at least 6 months, at room temperature, and thus, itcan be presented as an industrial product.

[0074] According to the present invention, a thick film can be formed bya single coating operation, and it is possible to provide a silica-basedcoating film which has a high cracking limit and which is excellent intransparency and has sufficient hardness. It can suitably be employed asa planarization film, or an interlayer insulation film or a protectivefilm in e.g. a semiconductor device or a liquid crystal display device.

1. A process for forming a silica-based coating film on a substratesurface, characterized by forming a reaction mixture comprising asilicon compound (A) represented by the formula (1): Si(OR)₄  (1)(wherein r is an alkyl group having from 1 to 5 carbon atoms) and/or asilicon compound (b) represented by the formula (2): R¹_(n)Si(OR²)_(4−n)  (2) (wherein R¹ is an unsubstituted or substitutedalkyl group having from 1 to 5 carbon atoms, an alkenyl group or an arylgroup, R² is an alkyl group having from 1 to 5 carbon atoms, and n is aninteger of from 1 to 3), an alcohol (C) represented by the formula (3):R³CH₂OH  (3) (wherein R³ is a hydrogen atom, or an unsubstituted orsubstituted alkyl group having from 1 to 12 carbon atoms), and oxalicacid (D), in such ratios that the alcohol (C) is from 0.5 to 100 molsper mol of all alkoxy groups contained in the silicon compounds (A) and(B), and the oxalic acid (D) is from 0.2 to 2 mols per mol of all alkoxygroups in the silicon compounds (A) and (B), and while maintaining themixture at a SiO₂ concentration of from 0.5 to 11% as calculated fromsilicon atoms in the mixture, by means of the alcohol (C), heating thereaction mixture at a temperature of from 50 to 180° C. until the totalremaining amount of the silicon compounds (A) and (B) in the reactionmixture becomes to be not more than 5 mol %, to form a solution of apolysiloxane having a number average molecular weight, as calculated aspolystyrene, of from 2,000 to 15,000 thereby formed, then applying acoating fluid containing the solution of the polysiloxane to a substratesurface, and thermally curing a coating film obtained by theapplication, at a temperature of from 80 to 600° C.
 2. A silica-basedcoating film having a film thickness of from 0.5 to 5 μm formed on asubstrate surface, obtained by forming a reaction mixture comprising asilicon compound (A) represented by the formula (1): Si(OR)₄  (1)(wherein R¹ is an alkyl group having from 1 to 5 carbon atoms) and/or asilicon compound (B) represented by the formula (2): R¹_(n)Si(OR²)_(4−n)  (2) (wherein R¹ is an unsubstituted or substitutedalkyl group having from 1 to 5 carbon atoms, an alkenyl group or an arylgroup, R² is an alkyl group having from 1 to 5 carbon atoms, and n is aninteger of from 1 to 3), an alcohol (C) represented by the formula (3):R³CH₂OH  (3) (wherein R³ is a hydrogen atom, or an unsubstituted orsubstituted alkyl group having from 1 to 12 carbon atoms), and oxalicacid (D), in such ratios that the alcohol (C) is from 0.5 to 100 molsper mol of all alkoxy groups contained in the silicon compounds (A) and(B), and the oxalic acid (D) is from 0.2 to 2 mols per mol of all alkoxygroups in the silicon compounds (A) and (B), and while maintaining themixture at a SiO₂ concentration of from 0.5 to 11% as calculated fromsilicon atoms in the mixture, by means of the alcohol (C), heating thereaction mixture at a temperature of from 50 to 180° C. until the totalremaining amount of the silicon compounds (A) and (B) in the reactionmixture becomes to be not more than 5 mol %, to form a solution of apolysiloxane having a number average molecular weight, as calculated aspolystyrene, of from 2,000 to 15,000 thereby formed, then applying acoating fluid containing the solution of the polysiloxane to a substratesurface, and thermally curing a coating film obtained by theapplication, at a temperature of from 80 to 600° C.
 3. A coating fluidcontaining a solution of a polysiloxane having a number averagemolecular weight, as calculated as polystyrene, of from 2,000 to 15,000,obtained by forming a reaction mixture comprising a silicon compound (A)represented by the formula (1): Si(OR)₄  (1) (wherein R is an alkylgroup having from 1 to 5 carbon atoms) and/or a silicon compound (B)represented by the formula (2): R¹ _(n)Si(OR²)_(4−n) (2) (wherein R¹ isan unsubstituted or substituted alkyl group having from 1 to 5 carbonatoms, an alkenyl group or an aryl group, R² is an alkyl group havingfrom 1 to 5 carbon atoms, and n is an integer of from 1 to 3), analcohol (C) represented by the formula (3): R³CH₂OH  (3) (wherein R³ isa hydrogen atom, or an unsubstituted or substituted alkyl group havingfrom 1 to 12 carbon atoms), and oxalic acid (D), in such ratios that thealcohol (C) is from 0.5 to 100 mols per mol of all alkoxy groupscontained in the silicon compounds (A) and (B), and the oxalic acid (D)is from 0.2 to 2 mols per mol of all alkoxy groups in the siliconcompounds (A) and (B), and while maintaining the mixture at a SiO₂concentration of from 0.5 to 11% as calculated from silicon atoms in themixture, by means of the alcohol (C), heating the reaction mixture at atemperature of from 50 to 180° C. until the total remaining amount ofthe silicon compounds (A) and (B) in the reaction mixture becomes to benot more than 5 mol %.
 4. A process for producing a coating fluidcontaining a solution of a polysiloxane having a number averagemolecular weight, as calculated as polystyrene, of from 2,000 to 15,000,which comprises forming a reaction mixture comprising a silicon compound(A) represented by the formula (1): Si(OR)₄  (1) (wherein R is an alkylgroup having from 1 to 5 carbon atoms) and/or a silicon compound (B)represented by the formula (2): R¹ _(n)Si(OR²)_(4−n)  (2) (wherein R¹ isan unsubstituted or substituted alkyl group having from 1 to 5 carbonatoms, an alkenyl group or an aryl group, R² is an alkyl group havingfrom 1 to 5 carbon atoms, and n is an integer of from 1 to 3), analcohol (C) represented by the formula (3): R³CH₂OH  (3) (wherein R³ isa hydrogen atom, or an unsubstituted or substituted alkyl group havingfrom 1 to 12 carbon atoms), and oxalic acid (D), in such ratios that thealcohol (C) is from 0.5 to 100 mols per mol of all alkoxy groupscontained in the silicon compounds (A) and (B), and the oxalic acid (D)is from 0.2 to 2 mols per mol of all alkoxy groups in the siliconcompounds (A) and (B), and while maintaining the mixture at a SiO₂concentration of from 0.5 to 11% as calculated from silicon atoms in themixture, by means of the alcohol (C), heating the reaction mixture at atemperature of from 50 to 180° C. until the total remaining amount ofthe silicon compounds (A) and (B) in the reaction mixture becomes to benot more than 5 mol %.